The detailed theory and operation of strain wave gearing are disclosed in U.S. Pat. No. 2,906,143 of Musser. As illustrated in FIGS. 1a-1b representing the prior art, a typical strain wave gearing 1 comprises a rigid circular spline 2, a flexible flexspline 3 disposed inside the circular spline 2, and a wave generator 4 of an elliptic configuration which fits in the flexspline 3 to deform the flexspline into an ellipsoid. The wave generator 4 includes an inner cam plate 5 of an elliptic profile and a flexible ball bearing 6 fittingly mounted on the periphery of the cam plate 5 to deform the bearing into an ellipsoid. The bearing 6 has an outer race 7 which fits in the flexspline 3. The flexspline is deformed into an ellipsoid by the wave generator 4 so that the flexspline is engaged with the circular spline 2 at two points on the major axis of the ellipsoid and the adjacent regions thereof. In FIG. 1a, the engagement points are illustrated as two points shown by arrows A and B. By virtue of the ball-bearing 6 of the wave generator 4, the ellipsoid of the flexspline 3 is rotated when the cam plate 5 is rotated, but the flexspline 3 is not directly rotated by the plate.
A strain wave drive with a modified type of gearing has been developed in the prior art as shown in FIG. 1b. The strain wave gearing 10 comprises a first circular spline 11 (also referred to herein as “dynamic spline”), a second circular spline 12 which is juxtaposed with the first circular spline along the axis of the first circular spline and having a different number of teeth from that of the first circular spline, a flexspline 13 disposed coaxially inside both circular splines and having the same number of teeth as the first circular (dynamic) spline, and a wave generator 14 for deforming the flexspline 13 into a non-circular, elliptical, configuration to bring the flexspline into partial engagement with the teeth of each of the first and second circular splines and for rotating the deformed configuration of the flexspline to produce a relative rotation between the first and second circular splines. The strain wave gearing 10 is made thin and flat as a whole because the circular splines 11 and 12 are juxtaposed with each other, and the flexspline is made in a circular shape. This type of strain wave gearing is referred to as “flat-shaped strain wave gearing” or “pancake” strain wave gear set.
In the flat-shaped strain wave gearing, the wave generator 14 deforms the flexspline 13 into an ellipsoid and rotates the deformed configuration of the flexspline. By the rotation of the wave generator 14, the engagement points of the flexspline 13 and the first circular spline 11, and the flexspline 13 and the second circular spline 12 are also rotated. As mentioned above, the number of teeth of the flexspline 13 is equal to that of the first circular or dynamic spline 11. Therefore, even if the configuration of the flexspline is rotated, there is no relative rotation between the flexspline 13 and the first circular spline 11. As the number of teeth of the flexspline 13 is different from that of the second circular (dynamic) spline 12, the first circular spline 11 is rotated relative to the second circular spline 12. Thus, in the case where an input shaft is mounted on the wave generator, one of the circular splines is fixed stationarily and an output shaft is attached to the other circular spline, so that relative rotation is obtained between the stationary part and the output shaft. For a further explanation of the flat-type strain wave gearing of the prior art, incorporation by reference is made to U.S. Pat. No. 4,974,470 to Ishikawa.
Another known type of strain wave gearing is cup-shaped strain wave gearing, in which a cup-shaped flexspline extends in a direction normal to the plane of the splines and one of the ends is closed to form a cup shape. Since the flexspline is made in a cup shape, however, it is disadvantageous in that the overall size of the gearing becomes larger and thus occupies more volume. However, their main advantage is that they are capable of being radially preloaded in order to reduce backlash at the meshing teeth. Cup type strain wave drives do not typically have a first circular or dynamic spline and the output is taken directly from the closed cup-shaped end of the flexspline. Thus, supporting the flexspline in a rigid manner becomes an important factor in the design of a housing for a drive train that incorporates a cup type strain wave drive. For a further explanation of the cup-shaped type of strain wave drives of the prior art, incorporation by reference is made to U.S. Pat. No. 4,823,638 to Ishikawa. Reference is also made to U.S. Pat. Nos. 8,776,638 and 8,661,940 of Ishikawa, and U.S. Published Application 2014-0150586 of Kanai, for detailed descriptions of recent improvements in the two types of strain wave drives.
It is known that the primary reason for failure of strain wave drives is ratcheting due to excessive torque loads. This is caused when the engaged gear teeth slip, i.e., ratchet, over each other, creating an offset between the centers of the drive's flexspline and the circular spline, that is, they become non-concentric. This, ultimately, results in failure of the drive from fatigue or, at minimum, a shortened life because of excessive teeth wear in mesh no longer as designed. It is therefore a primary object of the present invention to improve the torque capacity of a strain wave drive and to prevent ratcheting of the gearing.